Vitamin E (alpha-tocopherol) is an essential element in the mammalian diet since mammals can not synthesize plastoquinones or tocopherols. The first step in the formation of plastoquinones and tocopherols in plants is the formation of homogenistate from 4-hydroxyphenylpyruvate, a reaction catalyzed by 4-hydroxyphenylpyruvate dioxygenase (EC 1.13.11.27). Genetic mutants of this enzyme in Arabidopsis are deficient in both carotenoids and tocopherols (Norris et al. (1995) Plant Cell 7:2139-2149). Carotenoids (xanthopylls) in corn endosperm are valuable coloring agents in poultry feeds; tocopherols are antioxidants which may be important in oil stability and human health. Generally present at low levels in plant tissues, 4-hydroxyphenylpyruvate dioxygenase catalyzes a complex, irreversible reaction. Only recently has 4-hydroxyphenylpyruvate dioxygenase been purified to homogeneity from a plant source (Garcia et al. (1997 Biochem. J. 325:761-769). In plants, cDNAs encoding 4-hydroxyphenylpyruvate dioxygenase have been identified in carrots, barley and Arabidopsis thaliana with at least two different variants existing in this last plant. 4-Hydroxyphenylpyruvate dioxygenase is a known herbicide target (Mayonado et al. (1989) Pestic. Biochem. Physiol. 35:138-145; Schultz et al. (1993) FEBS lett. 318:162-166; Secor (1994) Plant Phys. 106:1429-1433). Even though plastoquinones and tocopherols are not synthesized by mammals and bacteria 4-hydroxyphenylpyruvate dioxygenase activity is present, often at high levels, and involved in phenylalanine and tyrosine degradation. Among others, the cDNAs encoding 4-hydroxyphenylpyruvate dioxygenase have been identified in Mycosphaerella graminicola, mice and Coccidiodes immitis. 
Plants synthesize alpha, beta, gamma and delta tocopherols of which alpha tocopherol (vitamin E) has the highest value for human nutrition. In soybean 7% of the tocopherols are vitamin E. Gamma tocopherol methyltransferase catalyzes the final step in vitamin E synthesis and has been purified to homogeneity from pepper, marigold, Euglena and spinach. The gene encoding gamma tocopherol methyltransferase from Synechocystis was identified by mutating an open reading frame encoding a methyltransferase located in the operon containing the 4-hydroxyphenylpyruvate dioxygenase gene in the Synechocystis genomic sequence (Shintani and Della Penna (1998) Abstract for the American Society of Plant Physiologists meeting in Madison, Wis.).
Since mammals can not synthesize tocopherols, the enzymes described here may be used for the discovery of new herbicides.